The surface of gram-negative bacteria, the outer membrane, is a mosaic of tightly associated lipopolysaccharide (LPS) and protein molecules. In some organisms, the outer membrane itself is covered by loosely bound polymers which comprise the bacterial capsule. It is the surface of the bacterium which interacts with the host in early stages of parasitism, and it is the host defenses directed towards surface constituents which play a role in the prevention, resolution or sometimes exacerbation of bacterial diseases. We are engaged in the study of the structure and function of nonprotein surface constituents of Neisseria gonorrhoeae and to a minor degree those of Chlamydia trachomatis and Taylorella equigenitalis (Contagious Equine Metritis Organism, CEMO). Those neisserial constituents of particular interest are the H8 antigen and the capsular polysaccharide. Our objectives are to isolate and characterize these surface antigens while determining their role in pathogenesis. H8, a surface antigen common to all pathogenic Neisseria, has been isolated from a strain of N. gonorrhoeae. Preliminary analyses have substantiated the unusual nature of this protease sensitive antigen. Unlike most outer membrane proteins characterized to date, it lacks aromatic amino acids as well as methionine and cysteine. Significant amounts of two unique fatty acids copurify with the otherwise homogeneous H8 preparation. The H8 antigens of all strains examined to date appear to be antigenically homogeneous, however, they are variably susceptible to proteolysis which suggests some structural variation in the molecule. The polysaccharide capsule has been isolated from two strains of gonococci. There appears to be strain variation with respect to the composition of the neutral sugars. This is reflected in antigenic variation. Although capsular material can be extracted from gonococci irrespective of piliation or opacity phenotype, pilated gonococci, thought to be the virulent phase of the organism, have 1 to 1.5 logs more extractable capsule compared to non-piliated gonococci.